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Re[2]: Basement Walls

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     Doesn't the basement floor provide a significant amount of lateral 
     resistance, therefore placing less importance on the footing?
     
     However, if backfilling is performed prior to the diaphragm being 
     installed, then the wall would experience the deflection associated 
     with a cantilevered retaining wall versus a simple span.  I have never 
     designed a residential basement wall, but it is hard to believe that a 
     wall with just a typical spread footing can develop enough resistance 
     against overturning without utilizing the weight of soil by extending 
     the footing farther into the soil.  Is the diaphragm generally 
     required to be in place prior to backfilling so that there is at least 
     spring resistance at the top of the wall.
     
     Incidentally, residential basement walls in this area (western New 
     York) are 6" or 8" thick unreinforced concrete or 8" unreinforced, 
     ungrouted CMU. The backfill for the walls is generally clay.  Although 
     cracking is a predominant problem, it does not seem like there are as 
     many actual failures of these walls as there should be.  This has been 
     puzzling to me for quite some time.  Can anybody explain this?
     
     John Hubert


______________________________ Reply Separator _________________________________
Subject: RE: Basement Walls
Author:  SpragueHO(--nospam--at)bv.com at Internet
Date:    8/7/97 1:41 


Scott,
     
You have raised some interesting issues.
     
Be careful about designing a basement wall as a retaining wall.  As a 
retaining wall the wall must be free to translate at the top.  Tension 
will occur on the soil side.  As a simple span wall tension occurs on 
the inside of the basement wall.  When the wall has 2 layers of rebar 
the design is easily accommodated.  With 1 layer (most common for 
residential) you have to decide where to place the rebar to be most 
effective.
     
I have designed them as both a retaining wall and a simple span for 
commercial buildings where the contractor wanted to back fill prior to 
the floor being placed.  In residential structures it is most common to 
design them as a simple span wall with a "generous" continuous spread 
footing to accommodate some degree of back fill.  Residential back fill 
is generally not compacted so that the applied back fill loads are not 
as great as commercial projects.  It is best to add a note to preclude 
back filling until the floor is in place.  Also be mindful of the 
details to transfer the forces into the diaphragm.
     
Regarding seismic soil loads on basement walls is a hole other topic. 
In the BSSC, I tried my best to see if we could get some consistency in
the geotechnical community on how to even calculate seismic induced soil 
loads and was met at the gate by an angry mob of geotechnical engineers 
carrying torches.  Some in the field say there is no seismic induced 
lateral soil load.  Others say that there is, and they give values.  As 
a structural engineer I would love to have a consensus among the 
geotechnical engineers on methodology, but if my experience in the BSSC 
is any indication that time is not soon.
     
Good luck,
Harold Sprague
Black & Veatch
BSSC TS 13 Chairman
 ----------
From: Horn
To: seaoc(--nospam--at)seaoc.org
Subject: Basement Walls
Date: Thursday, August 07, 1997 12:36AM
     
My brother is an Architect and we have been involved in the design of 
several custom homes.  We have an on-going argument about the design of 
basement walls.  He insists that homes the Salt Lake City area
     
with basements require only 24" wide footings for basements walls. 
Basically, the walls is pinned-pinned!  I've been able to be involved in 
homes with owners with enough money to not complain too much about the 
cost, and an Architect who responds well to some coaching.
     
     
     
I live in Illinois now and I've been looking at homes around here
(most have basements) and they do the same thing.  There's no way these 
walls are evaluated as retaining walls.  I've checked the loads on this 
type
of configuration and the shear that has to be dumped into the 
diaphragms is considerable.  The idea of preloading a wood diaphragm 
with large
amounts of deadload and then needing additional reserve capacity for 
seismic loads of the structure and additional soil lateral loads is not 
very
appealing to me.  I have submitted to considering the retaining wall as 
fixed at the base and pinned at the top in some instances.
     
     
     
Has anyone else been pressured to not design a basement wall as a 
retaining wall?  If there is anyone who has / does do this, please let 
me
know what the justification is.
     
     
     
Scott Horn